In a stationary type radiant ray CT (description will be made with respect to the case using an X-ray) called as the fourth generation type in which detectors are disposed along the entire circumference of a circle, an X-ray source is rotated about a reconstructive region for accommodating a body to be examined and the detectors are stationary. A gantry portion of such an apparatus is shown in FIG. 8. In the figure, reference numeral 1 denotes an image reconstructive region for reconstructing a cross-sectional image of the body to be examined disposed interiorly of the gantry portion, and 2 detectors for detecting the X-ray radiated toward the reconstructive region 1. An X-ray source 3 continuously rotates along a circle circumferntially of the reconstructive region 1 while generating a fan-shaped X-ray beam directed toward the reconstructive region 1. The X-ray beam is generated in pulsating manner when the X-ray source 3 is of a pulse type. When the X-ray source 3 is rotating for scanning, data from the respective detectors 2 are sampled at every predetermined angle in synchronization with the generation of the X-ray beam. This type is advantageous in that reliability in terms of mechanism is high because tomographing can be accomplished merely by rotating the X-ray source, a tomographing can be made at a high speed, and reduction of false image, such as a ring artifact, can effectively be achieved because each of the detectors 2 can equally pickup the imaging data in the reconstructive region 1.
In FIG. 9, there is shown a spatial relation between a scan and a data pickup portion in an X-ray CT apparatus. In this figure, the same portions as those shown in FIG. 8 are denoted by the same reference numerals or character. In FIG. 9, character O denotes the center of the reconstructive region 1, LA the circumference of a circle along which the detectors 2 are arranged. Each of the detectors 2 is arranged at an equi-interval and equi-angular-interval. The X-ray source 3 rotates along the circumference of the circle LB, and in the case of the pulse type X-ray source 3, the X-ray is emitted at the points of Xi (i=-n . . . 0 . . . n) or Yi (i= -n . . . 0 . . . n) on the circumference of the circle LB and transmissive data (view data) of the body under examination are collected by the detectors disposed in the positions receiving the fan-shaped X-ray. The data to be collected are limited to those in the range of an angle covering at least the reconstructive region 1.
In order to improve the resolution (spatial resolution) of the reconstructive image, particularly to improve the resolution in the radius direction, a so-called offset detection method has been employed. In this method, assuming that the X-ray beam is generated at the point X0, a straight line connecting the center of the detector D0 and the point X0 is intentionally displaced relative to a straight line connecting the point X0 and the center P of the reconstructive region, in other words, a geometrically identified position in such a relation is detected, whereupon the X-ray beam is generated. The data thus collected and the data collected in the point Y0 are in such a relation that the transmissive X-ray paths are not in coincidence with each other at the position in the vicinity of the center O of the reconstructive region 1. With such a method, it is contemplated to increase effective sampling data upon interpolation.
It is, however, extremely difficult to improve the resolution more than a limited amount. Although there are various methods, such as a method of performing the scanning with respect to a greater number of detectors arranged in higher density, or a method of performing a multi-rotational scanning while changing an amount of offset every rotation by the use of detectors having a narrow-width opening, the scan time is inadversely prolonged, the amount of exposure markedly increases, and an expensive apparatus is required.